1. The Field of the Invention
This invention relates to a method and apparatus for allocating bandwidth in a TDM network (e.g., STS-1s in a SONET (synchronous optical network)/SDH (synchronous digital hierarchy) network), amongst clients, on an in-service (i.e., without any interruption in the transport of data) and dynamic basis.
2. The Relevant Technology
Time Division Multiplexing (TDM) protocols such as SONET/SDH are used in optical fibre networks to transport data across the network. The SONET protocol enables data to be aggregated from several tributaries in a simple manner but the allocation or re-allocation of bandwidth between the tributaries is more difficult to accomplish with this protocol.
A SONET signal (i.e., such as STS-48, meaning 48 synchronous transport streams) is typically composed of multiple STS-1's which are assigned to various resources or clients, with the greater the number of STS-1's representing increased bandwidth. Advantageously, the assignment of bandwidth to each client is flexible in that it can be assigned and changed based on any number of applied control parameters. An aggregator aggregates (i.e., combines) the client STS-1's into a composite STS payload (e.g., STS-48 or STS-192) and the aggregate data stream is fed into a SONET transmit framer to insert section/line/path overhead information (data) and create a proper SONET frame.
The tributary data for SONET networks is typically encoded (for example, using HDLC) in such a manner as to differentiate between payload data and control codes used for signaling. In addition most encoding schemes are also encapsulation schemes in that they provide control codes for delineating the start and end of frames.
Typically, reallocating the STS-1's amongst the clients is done by re-provisioning, whereby the data channel between the clients of the transmit and receive nodes must be taken out of service, resulting in an interruption of communications and service. The proposed LCAS (Link Capacity Adjustment Scheme) standard for SONET provides a method for reallocating bandwidth using virtual concatenation but this method is relatively complex. Moreover, this method may not allow for optimum recovery in the event that a protection switch occurs during a reallocation of bandwidth.
Known methods for in-service bandwidth reallocation depend on the occurrence of synchronized events at each of the transmit and receive nodes of the data transport link (the transmit node being within a source device and the receive node being within a sink device). Consequently, since one such event (e.g., a reallocation “switch trigger”) might be lost or corrupted by or during a protection switch event, these methods are vulnerable to protection switching events which may produce severe data errors (resulting from an inability of the sink device to correctly extract the data) and/or interfere with successful completion of a reallocation process.
There is a need, therefore, for means to reallocate STS-1's amongst clients on an in-service and dynamic basis without interference by the occurrence of a protection switching event.